Ships and navigation

Roman merchant ships in the Mediterranean were sailing ships. They were often called corbitae, derived from corbis, "basket". The size of the ships was expressed in amphorae or grain-measures. A ship with a carrying capacity of 240 tons must have been about 10 metres wide and 30 metres long. A ship with a cargo of 450 tons was 10 to 12 metres wide and 35 to 40 metres long. We know through legal texts that the Romans were capable of building ships with a capacity of 300 to 400 tons, and perhaps even ships with a capacity of over 1000 tons were built. Technically speaking there were no obstacles. Underwater archaeology indicates that very large ships did indeed exist (especially for the transport of grain), but that smaller ships (70 tons or less), with a mixed cargo, were more common. The average capacity of ships sailing to Rome - an exceptional case - may have been about 250 tons.

Graffito of a ship called Europa, from the Casa della Nave Europa in Pompeii. Image: Langner 2001, nr. 2019, Taf. 129.

Orientation on the sea was based on the stars and, during the day, the characteristics of the land. Navigation in the Mediterranean Sea can be arranged so as to remain in sight of the mainland or of islands for most of the time. Characteristic mountains, promontories, and man-made structures such as temples and of course lighthouses served as beacons. On the other hand, sailing very close to the shore is dangerous, because the ship could run aground in bad weather.

For sailing the wind is of greater importance than the current. In the summer the common mediterranean winds usually come from the north, speeding up the journeys of ships going to the south. Ships going to the north had to travel twice as long. Some sailing times are mentioned in ancient literature, but these must be used with caution. Were these record times? What was the size of the ships, and the nature of the cargo? Alexandria - Puteoli: 15-20 days; Ostia - Carthago: 3- 5 days; Gades - Ostia: 9 days; Tarraco - Ostia: 6 days; Narbo Martius - Ostia: 3 days. For a distance along the coast of about 50 kilometres a small ship, with favourable winds and currents, would need about eight to ten hours (a land caravan consisting of beasts of burden would need at least two days).

The best sailing season was from late May to mid-September. Mid-March to late May, and mid-September to mid-November were considered reasonable periods. In the winter shipping came to an almost complete standstill. This period was called the mare clausum ("closed sea"). In this season the force of the gales was too great, and bad visibility at sea hampered navigation. Shipwreck or jettison were almost inevitable.

Transport routes in the Roman Empire. Image: Jean-Paul Rodrigue, The Geography of Transport Systems, New York 2020.